1. Field of the Invention
The invention relates to a method for coating solid pharmaceutical formulations by applying to said formulations a film comprised of a film-forming coating medium in the fluid state, the liquid phase of which medium is comprised primarily of water, and then drying the film. (The term "pharmaceutical formulations" should be understood to mean not only tablets, capsules, and dragees, but also pellets, globules, granules, crystals, and similarly compacted or agglomerated medicinal particles.)
2. Description of the Background
Aqueous coating media for solid pharmaceutical formulations have been in general use for a long time. A classical method for employing an aqueous coating is to coat a solid formulation with sugar solutions. Today this technique has been substantially replaced by other methods of film coating, i.e., "film drageeing". These film coating methods employ aqueous coating media comprising water-soluble cellulose derivatives, polyethylene glycols, polyvinyl alcohol, or polyvinyl pyrrolidone as binders or binder adjuvants, or organic coating media solutions.
Aqueous coating media have the advantage of being non-flammable, with non-explosive vapors, and the process of evaporating the water from the coating has no adverse effect on the environment. However, the water-soluble binders employed in known aqueous coating media are not entirely satisfactory. To the extent the coatings contain cellulose derivates as a binder, they are subject to attack by microorganisms, particularly at high humidity and temperature conditions, e.g. in tropical climates. Present aqueous coating solutions are often difficult to color, and require a high binder-to-pigment ratio. The dried films applied to solid pharmaceutical formulations result in coatings which are insufficiently glossy, have inadequate resistance to abrasion, and have poor storage stability. They are hygroscopic to some extent, and under high humidity tend to become adhesive, so that, e.g., the coated formulations may stick together.
A proven non-water-based coating medium comprises alcoholic solutions of acrylic and/or methacrylic polymers having side groups containing tertiary amino groups (Ger. Pat. No. 1,090,381). On solid pharmaceutical formulations these coating media produce smooth, shiny coatings which are non-hygroscopic and are not susceptible to microorganisms. In the acid medium of gastric juice the tertiary amino groups are converted to the salt form, in which form the polymers are readily water-soluble, so that the coating dissolves in the stomach in a few minutes. The sole disadvantage of this well-tried and tested coating medium is the fact that it contains flammable solvents, which must be recovered from the air outlet of the coating apparatus for reasons of safety and for the prevention of air pollution.
A water soluble binder has been sought for the preparation of aqueous coating media, which binder is superior to the known water-soluble binders in that aqueous solutions containing the binder can be highly colored if desired, and even when highly colored, said solution yields a coating with high gloss. Further, the binder sought should be superior in that a coating containing the binder on solid pharmaceutical formulations should be abrasion resistant, relatively non-brittle and non-hygroscopic, and not subject to microbiological attack. In addition, the resulting coating should be free from disagreeable tastes and odors, and of course should be nontoxic.